Broad band helical antenna

ABSTRACT

The invention relates to a broadband antenna having a first and a second helical antenna (203, 207) wherein the antennas (203, 207) have different resonance frequencies. An antenna which has essentially a larger bandwidth than the bandwidth of an individual helical antenna is formed in coaction between the helical antennas (203, 207), by arranging the antennas coaxially and in generally the same axial positions, wherein the helical antennas (203, 207) are separated galvanically, possibly with the exception of at respective supply points. When fitted to portable equipment, the helical antennas (203, 207) can be combined with an axially movable straight wire antenna.

BACKGROUND OF THE INVENTION

The present invention relates to a broadband antenna means forcasing-carried equipment.

Portable equipment, and then particularly hand-carried telephones, areoften provided with helical antennas. Such antennas are convenientbecause they can be made more robust and take up less space thanstraight wire antennas for instance. However, they do not always givesatisfactory antenna performance.

One problem is that a helical antenna is short and can therefore easilybe screened or obstructed by a user or by objects in its vicinity. Thehelical antenna is therefore often combined with an extendable straightwire antenna, which is not screened as easily and which also givesbetter antenna performance. For instance, the Patent Specifications U.S.Pat. No. 4,121,218, U.S. Pat. No. 5,204,687 and U.S. Pat. No. 4,868,576describe different antenna means for portable equipment in which ahelical antenna has been combined with an extendable straight wireantenna.

Another problem encountered with the use of helical antennas resides intheir limited bandwidth. This renders conventional helical antennasunusable in those cases when an antenna function having a very broadband is required, for instance in the JDC mobile telephone system. Incases such as these, it would be desirable to provide an antenna meanswhich has the positive properties of helical antennas coupled with abandwidth which is much greater than the bandwidth of a conventionalhelical antenna.

It is, of course, conceivable to provide portable equipment with twoseparate helical antennas which have mutually different resonancefrequencies. Such an antenna arrangement would be unnecessarily spaceconsuming, however.

The problems concerning bandwidth and space requirements are solved by abroadband antenna means according to the present invention. There isthus obtained a helical structure that is resonant at two frequencieswhich result from the different lengths of respective coils and whosedifference is due to the difference in the lengths of the coils.

SUMMARY OF THE INVENTION

The problems relating to screening of the short helical antennas issolved by combining the helical antennas with a straight wire antenna.

Further advantageous embodiments of the invention are defined in thedepending Claims. For instance, a helical structure which is resonant atmore than two frequencies can also be obtained in a similar manner, bycombining three or more coils.

It may also be problematic and require very high precision inmanufacture to achieve a well-defined mutual capacitance/inductancebetween the coils when they are mounted close to one another. This canbe alleviated by twisting the coils together prior to being wound totheir final helical shape, for instance.

BRIEF DESCRIPTION OF THE DRAWINGS

Two preferred embodiments of the invention will now be described in moredetail with reference to the accompanying drawings, in which

FIG. 1 is view of a first embodiment of the invention which includes,among other things, a first helical antenna incorporated in a sleeveprovided with an attachment means, a second helical antenna, an antennarod or pin which encloses a straight wire antenna and the second helicalantenna;

FIG. 2a illustrates a second embodiment of the invention which includes,among other things, a first helical antenna and a second helical antennaembodied in a sleeve provided with an attachment means, and an antennarod or pin which embraces a straight wire antenna;

FIG. 2b shows elements of the second embodiment illustrated in FIG. 2a;

FIG. 3 is a circuit diagram which illustrates an electric function ofthe helical antennas of the first embodiment illustrated in FIG. 1;

FIG. 4 is a circuit diagram which illustrates an electric function ofthe helical antennas of the second embodiment shown in FIGS. 2a and 2b;and

FIG. 5 shows three curves in a bandwidth diagram of a conventionalhelical antenna of a helical structure according to the invention, andof a straight wire antenna.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The antenna means illustrated in FIG. 1 is comprised of three maincomponents. Firstly, a first helical antenna which comprises a firstcoil 107 is embodied coaxially in a hollow, generally cylindricalinsulating sleeve 106 made of plastic material or some equivalentmaterial. One end of the coil 107 is connected to a conductive sleeve108, which is fastened in one end of the insulating sleeve 106. Theconductive sleeve 108 is effective in connecting the coil 107electrically to portable equipment, for instance a hand-carriedtelephone (not shown), and forms the means for attaching the antennameans externally to the portable equipment.

Secondly, a second helical antenna which is comprised of a second coil103 whose length is different to, and preferably shorter than the lengthof the first coil 107, and thirdly a straight wire antenna 105. Thesecond coil 103 is arranged coaxially in an insulated upper part 104 ofan antenna rod or pin 102 which is provided with a knob 101 on top ofthe pin. The lower part of the antenna pin includes the straight wireantenna 105, and the whole of the antenna pin 102 can be inserted andextended axially through the insulating sleeve 106, the first coil 107and the conductive sleeve 108.

Attached to the other end of the insulating sleeve 106 is an insulatingtube 109 which when fitted in place is located within the portableequipment and in which the antenna pin 102 is located in when in itsinserted or retracted position. Axial movement of the antenna pin ispreferably limited by the knob 101 and also by a shoulder or abutment(not shown) on the lower end of the antenna pin 102.

When the antenna pin 102 is telescoped to its inwardly insertedposition, the straight wire antenna will lie generally beneath theconductive sleeve 108 and is essentially inactive. The two coils 107,103 are located coaxially with one another in generally the same axialposition. The coils are therewith mutually coupledcapacitively/inductively, as shown in the circuit diagram of FIG. 3. Theresult is an antenna function which has a much broader band than theantenna function of each coil 107, 103 when taken individually.

In the outwardly extended position of the antenna pin 102, the straightwire antenna 105 will be positioned substantially above the conductivesleeve 108, wherein the straight wire antenna 105 is connected to theportable equipment either directly or via the first coil 107 and formsthe main antenna function.

The helical antennas and the straight wire antenna 105 can be connectedelectrically to circuits in the portable equipment galvanically and/orcapacitively/inductively in a combined form, wherein switching betweendifferent combinations is effected by extending or inserting the antennapin 102. Switch means intended for this purpose may conceivably beprovided in the proximity of the conductive sleeve 108, wherein theswitch means preferably coactable with parts of the antennas 103, 105that are located within the antenna pin 102.

The antenna means illustrated in FIG. 2a is comprised of two maincomponents. Firstly, a first helical antenna and a second helicalantenna which are comprised respectively of a first coil 207 and asecond coil 203 which are embodied coaxially in a hollow, generallycylindrical insulating sleeve 206 made of plastic material or someequivalent material. One end of respective coils 207, 203 is connectedto a conductive sleeve 208 which is attached to one end of theinsulating sleeve 206. The conductive sleeve 208 is operative inconnecting the coils 207, 203 electrically to portable equipment, forinstance a hand-carried telephone (not shown), and forms the means forattachment of the antenna means externally to the portable equipment.The first coil 207 and the second coil 203 are combined to form commonlya unitary helical form, which will be described in more detail belowwith reference to FIG. 2b.

Secondly, a straight wire antenna 205 is arranged in a lower part of anantenna pin or rod 202. The antenna pin 202 has an insulated upper part204 and carries a knob 201 at the top thereof. The whole of the antennapin 202 can be inserted and extended, i.e. telescoped, axially throughthe insulating sleeve 206, the first coil 207, the second coil 203 andthe conductive casing 208.

Attached to the other end of the insulating sleeve 206 is an insulatingtube 209 which when fitted in place is located within the portableequipment and in which the antenna pin 202 is located when in itsinserted position. Axial movement of the antenna pin is preferablylimited by the knob 201 and also by a shoulder or abutment (not shown)at the bottom end of the antenna pin 202.

When the antenna pin 202 is inserted, or retracted, the straight wireantenna will be located generally beneath the conductive sleeve 208 andis essentially inactive. The two coils 207, 203, in whose centers theupper insulating part 204 is located, are herewith active and connectedto the portable equipment in accordance with the circuit diagram shownin FIG. 4. The result is an antenna function which has a much broaderbandwidth than the antenna function of each coil 207, 203 per se.

When the antenna pin 202 is in its outwardly extended position, thestraight wire antenna 205 will be located generally above the conductivesleeve 208 and is connected to the portable equipment, either directlyor via the coils 207, 203, and forms the main antenna function.

The helical antennas and the straight wire antenna 205 can be connectedto circuits in the portable equipment galvanically and/orcapacitively/inductively in a combined form, wherein switching betweendifferent combinations is effected by extending and inserting theantenna pin 202 telescopically. In this regard, a switch means mayconceivably be provided in the proximity of the conductive sleeve 208,this switch means being capable of coacting with preferably the straightwire antenna 205 located in the antenna pin 202.

FIG. 2b shows in detail the insulating sleeve 206, the conductive sleeve208 and the coils 207, 203 of the two helical antennas. The Figureillustrates a suitable manner of arranging the first coil 207 and thesecond coil 203 in a common helical form whose diameter is slightlysmaller than the diameter of the insulating sleeve 206. In this case,the two coils 207, 203 consist of two insulated wires of mutuallydifferent lengths, for instance of the kind normally used intransformers.

The wire forming the first coil 207 and the wire forming the second coil203 are first twisted together along the full length of the shorterwire. The wires are then wound to the common helical form. Twisting ofthe wires together results in a well-defined capacitive/inductiveconnection between the wires. This obviates the need of producing thewinding with the same degree of precision as that required when thewires are wound parallel to one another in the common helical form. Thisgreatly facilitates manufacture.

The circuit illustrated diagrammatically in FIG. 3 is an electricalmodel of the antenna function of the first embodiment when the antennapin is in its inserted or retracted position, wherein the first and thesecond coil are described respectively by a first group 307 and a secondgroup 303, each having three series-connected inductances. Thecapacitance between the first and the second coil is shown with twocapacitances 310 connected in parallel at the interconnection points ofrespective inductances. One end of the first group 307 is parallelconnected directly to the circuit 312 in the portable equipment whileone end of the second group 303 is parallel connected to the circuit 312via a capacitance 311.

The circuit illustrated diagrammatically in FIG. 4 is an electricalmodel of the antenna function of the second embodiment when the antennapin is in its inserted or retracted position, or alternatively withoutthe antenna pin, wherein the first and the second coil are describedrespectively by a first group 407 and a second group 403, each havingthree series-connected inductances. The capacitance between the firstand the second coil is shown by two capacitances 410 connected inparallel at the interconnection points of respective inductances. Oneend of each group 407, 403 is directly parallel-connected to thecircuits 412 of the portable equipment.

The diagram shown in FIG. 5 illustrates the principle configuration ofthe standing wave ratio as a function of the frequency of differentantenna means. A curve 517 which describes a conventional helicalantenna has a narrow local minimum, which corresponds to a smallbandwidth. A curve 519 which describes a straight wire antenna has abroad local minimum, which corresponds to a large bandwidth. A curve 518which describes a helical structure according to the invention has twoseparate local minima located adjacent one another, which corresponds toa large total bandwidth. The marked frequency regions 515, 516 denotetransmission and reception bands for a broadband telephone system, forinstance JDC. Both of these frequency ranges 515, 516 are included bythe bandwidth of the inventive helical structure.

I claim:
 1. A broadband antenna means for equipment carried in a caseand intended to transmit and/or receive radio signals, said antennameans including a first helical antenna which is comprised of a firstcoil, and a second helical antenna which is comprised of a second coil,said coils having mutually different lengths which impart to respectivehelical antennas different resonance frequencies, said first coil andsaid second coil being conductively separated except for one end of eachof the coils and being disposed coaxially so as to form in coaction inessentially the same axial positions an antenna of essentially largerbandwidth than each of the helical antennas per se and being placedadjacent one another so that geometry thereof will generally coincide,with the exception of a section which generally corresponds to adifference between lengths of said coils; wherein an antenna rod isarranged to be axially extendable through the two coaxially arrangedcoils to reach a first position in which said antenna rod extendsoutside the case and has a conductive portion which is free from both ofthe coils, and a second position in which said conductive portion isinserted in the case and is free from said two coils and wherein theconductive portion of the antenna rod and the helical antennas areconnectable mutually in a combined form.
 2. The antenna means accordingto claim 1, said first coil and said second coil being twisted togetherwith the exception of the section that corresponds generally to thedifference between the lengths of said coils.
 3. The antenna meansaccording to claim 1, wherein at least one of said first and said secondcoils has an insulated outer layer.
 4. The antenna means according toclaim 1, wherein a first helical antenna supply point and a secondhelical antenna supply point are mutually connected conductively orcapacitively.
 5. The antenna means according to claim 4, wherein saidsupply point of the first helical antenna, said supply point of saidsecond helical antenna and said conductive portion of the antenna rodare mutually connectable conductively or capacitively/inductively in acombined form by axially moving said antenna rod.
 6. The antenna meansaccording to claim 1, said second helical antenna being arranged withinan upper insulating part of said antenna rod, which also includes saidconductive portion and is movable coaxially within said first helicalantenna, wherein said second helical antenna is located within saidfirst helical antenna when said antenna rod is in a retracted orinserted position.
 7. The antenna means according to claim 6, whereinsaid second helical antenna is essentially separatableelectromagnetically from said first helical antenna by axially movingsaid antenna rod.
 8. The antenna means according to claim 7, wherein asupply point of the first helical antenna, a supply point of the secondhelical antenna, and said conductive portion of the antenna rod aremutually connectable conductively or capacitively/inductively in acombined form by axially moving said antenna rod.
 9. A broadband antennameans for equipment carried in a case and intended to transmit and/orreceive radio signals, said antenna means including a first helicalantenna which is comprised of a first coil, and a second helical antennawhich is comprised of a second coil, said coils having mutuallydifferent lengths which impart to respective helical antennas differentresonance frequencies, said first coil and, said second coil beingconductively separated except for one end of each of the coils and beingdisposed coaxially so as to form in coaction in essentially the sameaxial positions an antenna of essentially larger bandwidth than each ofthe helical antennas per se and being twisted together with theexception of the section that corresponds generally to the differencebetween the lengths of said coils; wherein an antenna rod is arranged tobe axially extendable through at least one of the coaxially arrangedcoils to reach a first position in which said antenna rod extendsoutside the case and has a conductive portion which is free from both ofthe coils, and a second position in which said conductive portion isinserted in the case and is free from said two coils and wherein theconductive portion of the antenna rod and the helical antennas areconnectable mutually in a combined form.
 10. The antenna means accordingto claim 9, wherein at least one of said first and said second coils hasan insulated outer layer.
 11. The antenna means according to claim 9,wherein a first helical antenna supply point and a second helicalantenna supply point are mutually connected conductively orcapacitively.
 12. The antenna means according to claim 11, wherein saidsupply point of the first helical antenna, said supply point of saidsecond helical antenna and said conductive portion of the antenna rodare mutually connectable conductively or capacitively/inductively in acombined form by axially moving said antenna rod.
 13. The antenna meansaccording to claim 9, wherein said second helical antenna being arrangedwithin an upper insulating part of said antenna rod, which also includessaid conductive portion and is movable coaxially within said firsthelical antenna, wherein said second helical antenna is located withinsaid first helical antenna when said antenna rod is in a retracted orinserted position.
 14. The antenna means according to claim 13, whereinsaid second helical antenna is essentially separatableelectromagnetically from said first helical antenna by axially movingsaid antenna rod.
 15. The antenna means according to claim 14, wherein asupply point of the first helical antenna, a supply point of the secondhelical antenna, and said conductive portion of the antenna rod aremutually connectable conductively or capacitively/inductively in acombined form by axially moving said antenna rod.